Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. It is used widely for commercial and business applications for printing on various substrates from paper to cable marking or wide format vinyl sheeting and across markets ranging from industrial labelling to short-run printing to desktop document and pictorial imaging.
There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous inkjet, ink is supplied under pressure through orifices that produce jets of ink which break up into a continuous stream of droplets which may be of different sizes. The droplets are subsequently sorted such that some droplets form the image whereas others are caught and recirculated. For example, droplets can be selectively charged as a means of sorting or their size can be selectively varied to allow them to be sorted by selective deflection using a stream of air. The droplets that have been caught can then be recycled from the catcher and redispersed within the bulk ink.
In another process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant, which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in either case. Commonly used carrier media include water, organic solvents such as alcohols, ketones or hydrocarbons, as well as mixtures of water and organic co-solvents, such as alcohols, esters and ketones.
An important characteristic of inkjet printing is the need to control the ink on the surface of the substrate onto which it is deposited. In the case of common inkjet recording elements, an important factor in achieving this is the absorption of significant portions of the ink, particularly the carrier medium, into some part of the substrate structure. As a consequence, the printed image can appear to be dry immediately after printing and the absorbed liquid can evaporate later. This allows organic solvents and co-solvents with low boiling points to be usefully incorporated into ink formulations, particularly for drop-on-demand inkjet printing.
Inkjet printing in the home or small office involves printing aqueous inks onto porous substrates, such as paper, and is usually sufficiently slow to allow the ink to be absorbed into the substrate, thereby appearing to be almost instantly dry and not to require a drier. Commercial inkjet printing, such as is done for transactional or promotional applications, can however be very much faster and can involve the use of poorly-absorbing or even impermeable substrates. Such printers generally incorporate some means of immobilising, drying or otherwise solidifying the printed droplets to avoid undesirable droplet interactions.
One of the advantages of inkjet printing is that it is a non-contact method and can be used to print onto a wide range of surface topography. However, the nature of the surface, particularly its surface energy, can still present difficulties. The surface energy quantifies the disruption of chemical bonds that occurs when a surface is created. It is the interaction between the forces of cohesion and the forces of adhesion which determines whether or not wetting occurs. If complete wetting does not occur, then a bead of liquid will form with a contact angle which is a function of the surface energies of the system.
Successful printing is normally achieved by applying inks with a surface tension lower then the surface energy of the surface. Unfortunately water has a very high surface tension, which makes it particularly difficult to apply satisfactorily as droplets onto low energy, impermeable surfaces, such as plastic.
Thus, liquid absorption does not occur when printing onto impermeable substrates and in this case either a very fast drying process is applied, much more volatile organic solvents are used as a major component of the carrier medium, or the ink droplets undergo some kind of phase-change on the substrate. All of these practices have significant disadvantages. For example, many impermeable substrates are heat-sensitive, many volatile organic compounds raise concerns about health and safety and phase-change inks produce significantly thicker printed layers because most of the ink droplet is solidified.
The deposition of aqueous inkjet inks can therefore be problematical as they are either not capable of sufficiently wetting the substrate or do not dry quickly enough at the speeds used in inkjet printing, especially in continuous inkjet printing wherein the time between successive drops is very significantly shorter than for drop-on demand inkjet systems. As a result the ink droplets can wick, bleed or coalesce on some substrates, even some kinds of paper, and particularly on such impermeable substrates. Moreover, this tendency is exacerbated by the fast print speeds, high ink coverages, low surface energy surfaces and overlapping drops.
This problem can be avoided by using other kinds of ink, but these are usually non-aqueous. For example, non-aqueous radiation curable inks have been disclosed in patent application WO 99/07796, whereby the droplets are ‘cured’ or solidified by a chemical reaction initiated by an exposure, for example, to ultra-violet radiation. However the printed image may be insufficiently hardened or curing may continue after the initial curing time under the radiation source.
GB patent application No. 2390332 discloses a method of printing a plurality of radiation-curable droplet patterns, whereby the first pattern is immobilised before subsequent patterns are deposited. US patent application publication no. 2005/0185040A1 shows a means of immobilising droplets immediately downstream of a printing unit to avoid undesirable droplet interactions. FIG. 1 thereof shows the need for a plurality of ultra-violet laser scanners used with a plurality of printing units so that the droplets patterns are immobilised immediately after each deposition. Such ultra-violet laser scanners, which cause the ink droplets to solidify, are positioned immediately after a first printing unit and before a second printing unit, in what is referred to as an ‘interstation’ position. In another example, U.S. Pat. No. 4,939,992 shows the necessity to use forced hot-air interstation driers positioned after each of several successive flexographic printing units. U.S. Pat. No. 6,026,748 shows the necessity to use infra-red interstation driers positioned after each of several successive printing units.
An aqueous inkjet ink composition that can adhere to a wide range of surfaces, and in particular to impermeable substrates, is disclosed in WO 2008/075049, the disclosure of which is incorporated herein by reference. The compositions disclosed therein include discrete particles responsive to an external stimulus and having a lower viscosity in the printhead, such that the ink composition is conveniently jettable, and a higher viscosity in response to a stimulated change of conditions as the droplets are immobilised on the substrate.